The present invention relates to a protection circuit for a switched-mode power supply comprising a transformer with a primary winding and secondary windings, a switching transistor in series with said primary winding and a driver stage. Instead of the driver stage the power supply may have a control capacitor for the regulation. A switched-mode power supply of this kind is disclosed for example in EP-A-0 808 015.
In case of a short circuit the switching transistor of the power supply may remain in an unstable state not being completely closed during the off-interval. In this case the switching transistor may dissipate a power being to high which will damage it. This is especially the case for a self-oscillating switched mode power supply which has no driver circuit providing defined switching voltages and which may continue to oscillate in an undefined manner during said short circuit.